This invention relates to an automatic gain control (AGC) device for use in a direct conversion receiver for receiving a receiver input signal which comprises bursts in predetermined time slots.
In a radio communication network which may be a mobile communication network, a communication device comprises the receiver and a transmitter. A finite succession of time slots is used in transmitting a radio signal with one time slot of the succession selected from a predetermined number of time slots of each frame. The predetermined time slots are the time slots selected for the receiver input signal. In response to an information signal carried by the receiver input signal, the transmitter sends in general a response signal carried by a transmitter output signal which comprises bursts in particular time slots with each particular time slot selected in each frame from idle ones of the predetermined number of time slots.
Such a receiver comprises a receiving circuit for receiving the receiver input signal to produce a received baseband signal. Being used in the radio communication network, the receiver input signal is inevitably subjected to fading to reach the receiver with an input level which may vary as wide as some tens of decibels. The automatic gain control device is therefore supplied with the received baseband signal to produce a gain controlled signal having a controlled level which should be kept substantially constant.
In the receiver, an output circuit is supplied with the gain controlled signal to produce a receiver output signal representative of the information signal. In the case of FSK signal demodulation the output circuit comprises a frequency detector for frequency demodulating the gain controlled signal into a digital baseband signal and for producing a timing signal which indicates the time slots in each frame and the predetermined time slots. A digital data detector is supplied with the digital baseband signal and produces the receiver output signal.
In the automatic gain control device, an amplifier circuit is used in amplifying the received baseband signal into an amplified signal and a gain controlled by a gain control signal. The amplified signal unavoidably has a direct-current (d.c.) component. An a.c. coupling circuit is used as a direct-current blocking circuit for blocking the direct-current component to produce a direct-current component blocked signal which has a variable level and may be called a variable level signal. A level detector circuit is for detecting the variable level to produce a level value signal representative of level values of the variable level.
In a conventional automatic gain control device, the level value signal is supplied to the amplifier circuit as the gain control signal. It should be noted in this connection that the variable level signal has a variable direct-current offset which gives rise to a distortion in the receiver output signal. This is because the direct-current offset adversely affects frequency demodulation operation carried out by the frequency detector. Furthermore, the direct-current offset renders gain control operation of the automatic gain control device unstable. This is because it is difficult for the level detector circuit to discriminate between the direct-current offset and the variable level. The direct-current offset becomes serious when the amplifier circuit amplifies the received baseband signal with a high gain and with a small amount of gain control.
In principle, the direct-current blocking circuit removes the direct-current offset. The direct-current blocking circuit has an a.c. coupling time constant. The automatic gain control device is operable with an automatic gain control time constant. Usually, the coupling time constant is selected longer than the automatic gain control time constant so that the direct-current blocking circuit may sufficiently attenuate a variation caused by the gain control operation in a direct-current level of the variable level signal and so that the direct-current offset may not adversely affect both the frequence demodulation and the gain control operations.
The receiver input signal may be modulating according to frequency shift keying (FSK) at a certain bit or data rate. In this event, the a.c. coupling time constant must be selected dependent on the bit rate so as not to deteriorate a bit error rate of the receiver output signal. On the other hand, the automatic gain control time constant must be selected so as to sufficiently follow the fading which may rapidly vary. If the automatic gain control time constant is short, it becomes impossible for the direct-current blocking circuit to sufficiently attenuate the variation in the direct-current level.
The frequency shift keying may be minimum shift keying (MSK). According to the minimum shift keying, the receiver input signal is modulated with a small degree of modulation index, such as only 0.5. Under these circumstances, the direct-current offset brings forth serious adverse effects to the frequency demodulation and the gain control operations.